Apparatus and methods for detecting migration of neurostimulation leads

ABSTRACT

Apparatus and methods for detecting lead migration through the use of measured artifactual data about the tissue in the vicinity of the lead.

RELATED APPLICATION DATA

This application is a divisional application of U.S. patent applicationSer. No. 13/367,163, filed Feb. 6, 2012, which is a continuationapplication of U.S. patent application Ser. No. 11/096,483, filed Apr.1, 2005 (now U.S. Pat. No. 8,131,357), which applications are allincorporated herein by reference.

BACKGROUND OF THE INVENTIONS

1. Field of Inventions

The present inventions relate generally to neurostimulation systems.

2. Description of the Related Art

Neurostimulation systems, such as spinal cord stimulation (SCS) systems,deep brain stimulation systems and subcutaneous stimulation systems,include electrodes that are positioned adjacent to neural elements thatare the stimulation target. The electrodes are commonly mounted on acarrier and, in many instances, a plurality of electrodes are mounted ona single carrier. These carrier/electrode devices are sometimes referredto as “leads.” Because the proper placement of the electrodes iscritical to the success of neurostimulation therapy, the surgeon willcarefully position one or more leads such that the electrodes areadjacent to the target neural elements. There will typically be 1 to 5mm between adjacent leads.

Stimulation energy is delivered to the electrodes during and after theplacement process in order to verify that the electrodes are stimulatingthe target neural elements. Stimulation energy is also delivered to theelectrodes at this time to formulate the most effective stimulus pattern(or regimen). The pattern will dictate which of the electrodes aresourcing or returning current pulses at any given time, as well as themagnitude and duration of the current pulses. The stimulus pattern willtypically be one that provides stimulation energy to all of the targettissue that must be stimulated in order to provide the therapeuticbenefit (e.g. pain relief), yet minimizes the volume of non-targettissue that is stimulated. Thus, neurostimulation leads are typicallyimplanted with the understanding that the stimulus pattern will requirefewer than all of the electrodes on the leads to achieve the desired“paresthesia,” i.e. a tingling sensation that is effected by theelectrical stimuli applied through the electrodes.

A wide variety of leads have been introduced. One common type ofneurostimulation lead is the “in-line” lead, which includes a pluralityof spaced electrodes on a small diameter carrier. In-line leads arerelatively easy to place because they can be inserted into the spinalcanal through a percutaneous needle in a small locally-anesthetizedincision while the patient is awake and able to provide feedback.In-line leads are also advantageous because they can be removedrelatively easily. One of the disadvantages of in-line leads is thatthey are prone to migrating in the epidural space, either over time oras a result of a sudden flexion movement.

Lead migration can result in the targeted neural elements no longerbeing appropriately stimulated and the patient no longer realizing thefull intended therapeutic benefit. Lead migration is, however, not theonly reason that the therapeutic effects of a previously effectiveneurostimulation regimen will diminish or simply disappear, which canmake diagnosis difficult. Moreover, even after a physician hasdetermined that lead migration has occurred and that the system must bereprogrammed to accommodate the new positions of the electrodes,conventional neurostimulation systems do not provide the physician withinformation about the movement of an individual lead, such as how farthe lead has moved relative to the underlying tissue. This makesreprogramming especially difficult because it relies on trial and errorand patient feedback to identify which of the lead electrodes are nowaligned with the target neural elements and which are not.

The present inventors have also determined that conventional methods ofdetecting the relative positions of two or more neurostimulation leadsat the time of implantation, as well as at subsequent times, aresusceptible to improvement.

SUMMARY OF THE INVENTIONS

Apparatus and methods in accordance with one of the present inventionsmeasure artifactual data about tissue in the vicinity of an implantedlead, such as a neurostimulation lead, to detect lead migration and, insome implementations, to provide information about the lead migration.Such artifactual tissue data includes tissue impedance data andphysiologically evoked potential data. Typically, the baselineartifactual data will be measured when the system is providing thedesired therapeutic result and the subsequent artifactual data will bemeasured thereafter. Variations from the baseline artifactual data maybe used to indicate that the lead has migrated.

Such apparatus and methods are advantageous for a variety of reasons.For example, the apparatus and methods provide a reliable indicationthat a particular lead (or leads) in a neurostimulation system hasmigrated. The present inventions also provide specific information aboutthe migration, such as the magnitude and direction of the migrationrelative to the underlying tissue, which reduces the difficultyassociated with system reprogramming.

Apparatus and methods in accordance with one of the present inventionsdetermine the relative positions of neurostimulation leads by measuringthe impedance of tissue between each neurostimulation lead and anelectrode that is located in spaced relation to the neurostimulationleads. The measurements may be compared to determine whether one of theneurostimulation leads is a greater distance from the electrode than theother.

Such apparatus and methods are advantageous for a variety of reasons.For example, the apparatus and methods provide a convenient way todifferentiate the leads from one another during the post-implantationprogramming process.

The above described and many other features of the present inventionswill become apparent as the inventions become better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Detailed description of exemplary embodiments of the inventions will bemade with reference to the accompanying drawings.

FIG. 1 is a side view of a neurostimulation system in accordance withone embodiment of a present invention.

FIG. 1A is an end view of an implantable pulse generator in accordancewith one embodiment of a present invention.

FIG. 2 is a functional block diagram of an implantable pulse generatorin accordance with one embodiment of a present invention.

FIG. 3 is an illustration of a stimulation pulse that may be produced bythe implantable pulse generator illustrated in FIG. 2.

FIG. 4 is a functional block diagram of an implantable pulse generatorin accordance with one embodiment of a present invention.

FIG. 5 is an illustration of a stimulation pulse that may be produced bythe implantable pulse generator illustrated in FIG. 4.

FIGS. 6A and 6B are plan views showing exemplary implantable leads inbaseline positions and subsequent positions.

FIG. 7 is a graph showing impedance measurements taken when theimplantable leads are in the positions illustrated in FIGS. 6A and 6B.

FIGS. 8A and 8B are plan views showing exemplary implantable leads inbaseline positions and subsequent positions.

FIG. 9A is a graph showing evoked potential measurements taken at one ofthe implantable leads when the lead is in the positions illustrated inFIGS. 8A and 8B.

FIG. 9B is a graph showing evoked potential measurements taken at theother of the implantable leads when the lead is in the positionsillustrated in FIGS. 8A and 8B.

FIG. 10 is a flow chart summarizing various processes in accordance withthe present inventions.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

This application is related to concurrently filed application Ser. No.11/096,662, entitled “Apparatus and Methods for Detecting Position andMigration of Neurostimulation Leads.”

The following is a detailed description of the best presently knownmodes of carrying out the inventions. This description is not to betaken in a limiting sense, but is made merely for the purpose ofillustrating the general principles of the inventions. The detaileddescription is organized as follows:

I. Exemplary Neurostimulation Systems

II. Exemplary Lead Migration Detection

III. Exemplary Corrective Measures

The section titles and overall organization of the present detaileddescription are for the purpose of convenience only and are not intendedto limit the present inventions.

I. Exemplary Neurostimulation Systems

The present inventions have application in a wide variety ofneurostimulation systems. Although the present inventions are not solimited, examples of such systems are illustrated in FIGS. 1-5.Referring first to FIGS. 1 and 1A, an exemplary implantableneurostimulation system 100 includes first and second implantable leads102 and 104. The exemplary leads 102 and 104 are in-line leads and, tothat end, both of the leads consist of a plurality of in-line electrodes106 carried on a flexible body 108. In the illustrated embodiment, thereare eight (8) electrodes on lead 102, which are labeled E1-E8, and thereare eight (8) electrodes on lead 104, which are labeled E9-E16. Theactual number of leads and electrodes will, of course, vary according tothe intended application and the present inventions are not limited toany particular numbers of leads and electrodes. The leads 102 and 104may be implanted into a desired location, such as adjacent to thepatient's spinal column, through the use of an insertion needle or otherconventional techniques. Once in place, the electrodes 106 may be usedto supply stimulation energy to the target neural elements or othertarget tissue.

The exemplary neurostimulation system 100 illustrated in FIGS. 1 and 1Aalso includes an implantable pulse generator (IPG) 110 that is capableof directing tissue stimulation energy to each of the electrodes 106. Tothat end, each of the electrodes 106 on the lead 102 is electricallyconnected to the IPG 110 by a respective signal wire 112 (some of whichare not shown) that extends through, or is imbedded in, the associatedflexible body 108. Similarly, the electrodes 106 on the lead 104 areelectrically connected to the IPG 110 by respective signal wires 114(some of which are not shown). The signal wires 112 and 114 areconnected to the IPG 110 by way of an interface 115. The interface 115may be any suitable device that allows the leads 102 and 104 to beremovably or permanently electrically connected to the IPG 110. Such aninterface may, for example, be an electro-mechanical connectorarrangement including lead connectors 117 a and 117 b within the IPG 110that are configured to mate with corresponding connectors (onlyconnector 119 a is shown) on the leads 102 and 104. Alternatively, theleads 102 and 104 can share a single connector that mates with acorresponding connector on the IPG. Exemplary connector arrangements aredisclosed in U.S. Pat. Nos. 6,609,029 and 6,741,892, which areincorporated herein by reference.

The exemplary IPG 110 includes an outer case 116 that is formed from anelectrically conductive, biocompatible material such as titanium and, insome instances, will function as an electrode. The IPG 110 is typicallyprogrammed, or controlled, through the use of an external(non-implanted) programmer 118. The external programmer 118 is coupledto the IPG 110 through a suitable communications link, which representedby the arrow 120, that passes through the patient's skin 122. Suitablelinks include, but are not limited to, radio frequency (RF) links,inductive links, optical links and magnetic links. The programmer 118 orother external device may also be used to couple power into the IPG 110for the purpose of operating the IPG or replenishing a power source,such as a rechargeable battery, within the IPG. Once the IPG 110 hasbeen programmed, and its power source has been charged or otherwisereplenished, the IPG may function as programmed without the externalprogrammer 118 being present.

With respect to the stimulus patterns provided during operation of theexemplary neurostimulation system 100, electrodes that are selected toreceive stimulation energy are referred to herein as “activated,” whileelectrodes that are not selected to receive stimulation energy arereferred to herein as “non-activated.” Electrical stimulation will occurbetween two (or more) electrodes, one of which may be the IPG case, sothat the electrical current associated with the stimulus has a paththrough the tissue from one or more electrodes configured as anodes toone or more electrodes configured as cathodes, or return electrodes. Thereturn electrode(s) may be one or more of the electrodes 106 on theleads 102 and 104 or may be the IPG case 116. Stimulation energy may betransmitted to the tissue in monopolar or multipolar fashion. Monopolarstimulation occurs when a selected one of the lead electrodes 106 isactivated along with the case 116. Bipolar stimulation occurs when twoof the lead electrodes 106 are activated. For example, electrode E3 onlead 102 may be activated as an anode at the same time that electrodeE11 on lead 104 is activated as a cathode. Tripolar stimulation occurswhen three of the lead electrodes 106 are activated. For example,electrodes E4 and E5 on lead 102 may be activated as anodes at the sametime that electrode E13 on lead 104 is activated as a cathode. Generallyspeaking, multipolar stimulation occurs when multiple lead electrodes106 are activated.

Turning to FIG. 2, the exemplary IPG 110 has a plurality of dual currentsources 124. Each dual current source 124 includes a positive currentsource that can function as an anode (+I1, +I2, +I3, . . . +Icase) to“source” current to a load, as well as a current source that canfunction as a cathode (−I1, −I2, −I3, . . . −Icase) to “sink” currentfrom the load, through a common node 126. The load is the tissue thatresides between the activated electrodes 106, the wires (and otherconductive elements), and the coupling capacitor (C1, C2, C3, . . .Ccase) that connects the associated electrode to the common node 126 ofthe dual current source 124.

The IPG programming will dictate which of the electrodes, i.e. the leadelectrodes 106 and the IPG case 116, will act as sources and sinks atany particular time. To that end, the IPG 110 is provided with aprogrammable current control circuit 128 that causes selected dualcurrent sources 124 to operate as an anode or a cathode, at specifiedtimes, to source or sink current having predetermined amplitude. In theillustrated embodiment, where there are eight (8) electrodes 106 on lead102 (labeled E1-E8), eight (8) electrodes on lead 104 (E9-16), and anIPG case 116 that can function as an electrode (labeled Ecase), thereare seventeen individually operable dual current sources 124. Thecontrol circuit 128, which typically operates in accordance with storedcontrol data that is received from the programmer 118, also turns offthe selected dual current sources 124 at specified times. Alternativeimplementations may, however, employ fewer dual current sources thanthere are electrodes. Here, at least some of the dual current sourceswill be connected to more than one electrode through a suitablemultiplexer circuit. Alternative implementations may also be configuredsuch that the IPG case 116 only functions as an anode, or such that theIPG case only functions as a cathode

The control circuit 128 may, in addition, be used to perform variousmeasurement functions. For example, the control circuit 128 may be usedto measure the electrode voltage V_(E1), V_(E2), V_(E3) . . . V_(E16) atthe output node 126 of each dual current source 124, whether theelectrode is activated or non-activated. This allows the electrodevoltage at the electrode to be measured which, in turn, facilitatesimpedance measurements.

The operation of the control circuit 128 may be explained in the contextof the following example. Referring to FIG. 2, the control circuit 128may be used to simultaneously turn on (or enable) the positive currentsources in the dual current sources 124 connected to lead electrodes E1and E2 during time T1. The negative current source in the dual currentsource 124 connected to lead electrode E9 is also turned on during timeT1. All other current sources are off (or disabled) during the time T1.This causes electrodes E1 and E2 to be activated as anodes at the sametime that electrode E9 is activated as a cathode. Currents +I1 and +I2are sourced from electrodes E1 and E2 at the same time that current −I9is sunk into electrode E9. The amplitudes of the currents +I1 and +I2may be any programmed values, and the amplitude of the current −I9should be equal to −(I1+I2). That is, the current that is sourced isequal to the current that is sunk. After time period T1, the controlcircuit 128 will typically switch the polarities of the electrodes E1,E2 and E9 during a second time period T2 so that the electrodes E1 andE2 will be activated as cathodes and the electrode E9 will be activatedas an anode.

Operating the control circuit 128 in this manner produces the biphasicstimulation pulse 130 illustrated in FIG. 3 that is characterized by afirst phase (period T1) of one polarity followed by a second phaseimmediately or shortly thereafter (period T2) of the opposite polarity.The electrical charge associated with the first phase should be equal tothe charge associated with the second phase to maintain charge balanceduring the stimulation, which is generally considered an importantcomponent of stimulation regimes, although this is not required by thepresent inventions. Charge balance of the biphasic stimulation pulse 130may be achieved by making the amplitudes of the first and second phases,as well as the periods T1 and T2, substantially equal. Charge balancemay also be achieved using other combinations of phase duration andamplitude. For example, the amplitude of the second phase may be equalto one-half of the amplitude of the first phase and the period T2 may beequal to twice the period T1.

Neurostimulation systems in accordance with the present inventions mayalso employ the alternative IPG 110′ illustrated in FIG. 4, whichincludes a plurality of dual voltage sources 124′ that are respectivelyconnected to the lead electrodes E1-E16 and the IPG case electrodeEcase. Each dual voltage source 124′ applies a programmed voltage to theassociated electrode when turned on by way of a node 126′ and a couplingcapacitor (C1, C2, C3, . . . Ccase). Alternative implementations may,however, employ fewer dual voltage sources than there are electrodes.Here, at least some of the dual voltage sources will be connected tomore than one electrode through a suitable multiplexer circuit. Aprogrammable voltage control circuit 128′ controls each of the dualvoltage sources 124′ and specifies the amplitude, polarity and durationof the voltage that is applied to the electrodes.

The dual voltage sources 124′ and control circuit 128′ may be used toproduce the biphasic stimulation pulse 130′ illustrated in FIG. 5 thatis characterized by a first phase (period T1) of one polarity followedby a second phase immediately or shortly thereafter (period T2) of theopposite polarity applied between any two electrodes. Charge balance ofthe biphasic stimulation pulse 130′ may be achieved by making theamplitudes of the first and second phases, as well as the periods T1 andT2, equal. Charge balance may also be achieved using other combinationsof phase duration and amplitude. For example, the amplitude of thesecond phase may be equal to one-half of the amplitude of the firstphase and the period T2 may be equal to twice the period T1. The controlcircuit 128′ may also be used to measure the current flowing to or fromeach electrode, whether the electrode is activated or not, as well aselectrode voltage (E_(V1)-E_(V16)) appearing at the common node 126′ ofeach non-activated dual voltage source 124′. These current and voltagemeasurements also facilitate impedance measurements.

Additional details concerning the above-described and other IPGs may befound in U.S. Pat. No. 6,516,227 and U.S. Pub. App. 2003/0139781, whichare incorporated herein by reference. It also should be noted that theblock diagrams illustrated in FIGS. 2 and 4 are functional diagrams, andare not intended to limit the present inventions to any particular IPGcircuitry.

II. Exemplary Lead Migration Detection

As noted above, neurostimulation systems in accordance with the presentinventions are capable of detecting post-implantation migration of theleads 102 and 104 (i.e. movement of the leads relative to the underlyingtissue) and, in some implementations, are also capable of providinginformation about the migration. Generally speaking, the presentmigration detection processes employ artifactual tissue measurements,such as tissue impedance measurements or evoked potential measurements,to detect migration of the leads 102 and 104. The data gleaned from themeasurements may be stored and processed by the IPG control circuit 128(or 128′), by the external programmer 118, by some combination thereof,or the like. Additionally, the artifactual tissue measurements (e.g.,tissue impedance measurements and evoked potential measurements) allowlead migration to be detected on an individual basis relative to theunderlying tissue, as opposed to detecting migration of one leadrelative to the other. Accordingly, the processes described herein maybe employed in neurostimulation systems with a single lead as well asneurostimulation systems, such as the exemplary system 100, with aplurality of leads.

Turning first to tissue impedance, the impedance of tissue adjacent tothe lead electrodes 106 may be measured and monitored in order toprovide an indication that a lead has migrated. The tissue impedancemeasured at the lead electrodes 106 depends on the tissue in vicinity ofthe electrode (i.e. tissue within about 1 mm of the electrode) asopposed to all of the tissue located between electrodes on adjacentleads. Such tissue includes fat, collagen, bone, ligament, the white andgray matter of the spinal cord, and dura. The measured impedance willalso typically vary from electrode to electrode along the length of alead.

Referring first to FIG. 6A, after the leads 102 and 104 have beenproperly positioned within tissue T (“proper” positioning varies frompatient to patient), the impedance of the tissue adjacent to each of theelectrodes 106 (individually identified as E1-E16) may be measured. Thismay be accomplished in a variety of ways. For example, impedance may bedetermined by sourcing current from the IPG outer case 116, whichfunctions as electrode Ecase (FIG. 2), and sinking current at a selectedone of the electrodes E1-E16 at a predetermined voltage. Given that mostof the drop will occur at the smaller electrode, and that the leadelectrodes 106 are much smaller than the IPG outer case 116, it can beassumed that the measured impedance between the selected lead electrodeand the outer case is primarily due to the impedance of the tissueadjacent to the selected lead electrode. It should be noted that thecurrent used for impedance measurements is a sub-threshold current pulse(e.g. 1 mA for 20 μs) that will not cause stimulation or substantiallydrain the IPG battery. This process may be repeated for each of theelectrodes E1-E16. Alternatively, an electrode that is sutured anywherein or on the patient's body and, preferably, that is larger than thelead electrodes, may be used in place of the case IPG outer case forthese measurements.

Preferably, the impedances will be measured at each of the electrodesE1-E16 immediately after the lead has been implanted and theneurostimulation system IPG control circuit 128 (or 128′) has beenprogrammed to produce the desired therapeutic effect. Such an impedancemeasurement, which is referred to herein as a “baseline impedancemeasurement,” may consist of a single set of measurements at electrodesE1-E16 or a number of sets of measurement at the electrodes that areaveraged together on an electrode-by-electrode basis. Exemplary plots ofthe impedance at each electrode E1-E8 on lead 102 and each electrodeE9-E16 on lead 104 are shown with solid lines in FIG. 7. Baselineimpedance measurements may also be trended in the manner describedbelow.

Impedance measurements are also taken at each of the electrodes E1-E16at various times after the baseline impedance measurement has beenestablished. For example, impedance measurements may be taken at aperiodic check-up or in response to an indication from the patient thatthe neurostimulation system is no longer providing the same level oftherapeutic effect that it did when the baseline impedance measurementwas taken. Such an impedance measurement is referred to herein as a“subsequent impedance measurement” and may consist of a single set ofmeasurements at electrodes E1-E16 or a number of sets of measurements atthe electrodes that are averaged together on an electrode-by-electrodebasis. The subsequent impedance measurement is compared to the baselineimpedance measurement to determine whether or not the measurements havechanged. A change may indicate that the associated lead has migrated. Afeature comparison analysis, such as a peak comparison, a slopecomparison, or the like, may be used to determine the magnitude anddirection of any detected lead migration. One example of a mathematicaltechnique that may be used to implement the feature comparison analysisis a cross-correlation function. Other suitable algorithms may also beused to implement the feature comparison analysis.

Baseline impedance measurements include non-trended baseline impedancemeasurements, where the baseline impedance values do not change overtime, and trended baseline impedance measurements, where the baselineimpedance values are adjusted so as to account for factors such astissue necrosis and fibrosis that cause the measured impedance to changeslowly over time, as compared to lead migration, which tends to produceabrupt changes in impedance measurements. A trended baseline impedancevalue may be established per the following examples. When a subsequentimpedance measurement is substantially similar to the baseline impedancemeasurement (based on, for example, a feature comparison analysis), itmay be assumed that the difference in the impedance measurement is notdue to lead migration. The substantially similar subsequent impedancemeasurement may then be used to establish a new baseline impedancemeasurement value. This may be accomplished by substituting thesubstantially similar subsequent impedance measurement for the originalbaseline impedance measurement or by averaging the two. Additionally, oralternatively, as additional subsequent impedance measurements are takenover time, a moving average process may be used to establish the trendedbaseline impedance value.

Exemplary positions of the leads 102 and 104 at the time of a subsequentimpedance measurement are illustrated in FIG. 6B and the correspondingplots of the subsequent impedance measurement at each of the electrodeE1-E16 is shown with dashed lines in FIG. 7. Referring first to lead102, the plot of the subsequent impedance measurement is clearlydifferent than the plot of the baseline impedance measurement. Thisdifference provides an indication that the lead 102 may have movedrelative to the tissue in the vicinity of the lead. Additionalinformation that can confirm movement may be obtained by comparing theplots, or the before and after tissue impedance measurements at eachelectrode E1-E8, to one another. In the illustrated example, thesubsequent tissue impedance measurements taken at electrodes E2 and E3are essentially the same as the baseline tissue impedance measurementstaken at electrodes E4 and E5, respectively. From this, it may beinferred that the electrodes E2 and E3 have moved to the locationsoriginally occupied by electrodes E4 and E5, respectively, and that thelead 102 has moved a distance corresponding to two electrodes 106. If,for example, there is a 4 mm electrode-to-electrode spacing, it could beinferred that the lead 102 moved 8 mm.

Turning to lead 104, the plot of the subsequent impedance measurement isessentially the same as the plot of the baseline impedance measurement.This provides an indication that the lead 104 has not moved relative tothe tissue.

Other information may also be gleaned from comparisons of the subsequentimpedance measurement and the baseline impedance measurement. Forexample, in some instances, the lowest of the baseline impedancemeasurements or an average of all of the baseline impedance measurementsmay be used to establish a “nominal impedance value.” If the nominalimpedance value increases after the lead 102 has moved, it may beinferred that the distance between the lead 102 and the IPG outer case116 has increased. Similarly, if the nominal impedance value decreasesafter the lead 102 has moved, it may be inferred that the distancebetween the lead 102 and the IPG outer case 116 has decreased.

Impedance measurements may also be used to determine information that isneeded prior to programming or reprogramming. The mean impedance valuefor each lead 102 and 104 (i.e. the average value of each of theimpedance measurements taken at the electrodes 106 on the lead) willtypically be greater for the lead that is the greatest distance from theIPG case 116 (or 116′). As such, the mean impedance measurements mayalso be used to determine the relative positions of the leads 102 and104 with respect to the IPG 110 (or 110′). This relative positiondetermination will typically be made when the leads 102 and 104 areinitially implanted into the patient to assist with the programmingprocess, but may also be made at other times if needed.

In the SCS context, the leads 102 and 104 may be implanted, for example,anywhere from the cervical area down to the sacral area. The leads maybe positioned such that one lead is in one area and one lead is in adifferent area, such that both leads are located in the same area andare aligned with one another with respect to their distance from the IPGcase 116 (or 116′), or such that both leads are located in the samearea, but are staggered with respect to their distance from the IPGcase. As noted above, it may be inferred that the lead which producesthe largest mean impedance value is the greatest distance from the case.If, for example, the IPG 110 (or 110′) is implanted in the upper regionof a buttock, one lead is positioned in the thoracic area and one leadis positioned in the cervical area, then it may be inferred that thelead which produces the larger mean impedance value is in the cervicalarea. If both of the leads are positioned within the same area (e.g. thecervical area or the thoracic area), it may be inferred that the leadsare aligned with one another if the mean impedance values aresubstantially equal, or it may be inferred that the leads are staggeredand that the lead with the greater value is a greater distance from theIPG 110 (or 110′) if the mean impedance values are substantiallydifferent.

Tissue impedance measurements may also be taken in essentiallycontinuous fashion after the baseline impedance measurement has beenestablished. For example, the therapeutic pulses in a SCS program aretypically 1 ms in duration and are supplied at a frequency of 50 Hz.There are, therefore, 19 ms between each therapeutic pulse during whichtime sub-therapeutic pulses may be provided for impedance measurementpurposes. The continuously-taken subsequent impedance measurements maybe compared to the baseline impedance measurement as they are taken inorder to provide a real time indication that a lead has moved. Once sucha determination has been made, corrective measures such as thosediscussed in Section III below may be automatically undertaken or thepatient may simply be advised of the situation so that he or she canrelay this information to a physician.

Evoked potential measurements (also referred to herein as“physiologically evoked potential measurements”), which generallyinvolve providing a stimulation pulse to nerves at one electrode andmeasuring the generated action potential at another electrode, may alsobe used to determine whether or not a lead has migrated relative to theunderlying tissue in the vicinity of the lead. Referring first to FIG.8A, baseline evoked potential measurements may be taken after theimplantable leads 102 and 104 have been properly positioned adjacent tothe tissue T (“proper” positioning varies from patient to patient). Withrespect to lead 102, a stimulation pulse will be supplied by electrodeE1 to the adjacent tissue and the resulting action potential willgenerate a measurable deviation in the voltage that the programmablecontroller 128 sees at electrode E2 for a finite time that can bemeasured (or “read”). A suitable stimulation pulse for this purpose is,for example, 4 mA for 200 μs. Such stimulation is preferablysupra-threshold, but not uncomfortable. This process may be performedonce, or many times and averaged (e.g. 100 times), so that “baseline”stimulate-E1/evoked-potential-at-E2 data may be generated and measured,and then plotted in the manner illustrated for example in FIG. 9A (notethe solid line). The deviation in voltage caused by the evoked potentialdue to the stimulation pulse at electrode E1 could also be sequentiallymeasured at electrodes E3-E8 (either once or many times and averaged) inorder to provide additional baseline data and plots. Electrodes E2-E9may also be used to supply stimulation pulses and evoked potentials maybe measured at one or more of the other electrodes on lead 102.

Turning to lead 104, a stimulation pulse may be supplied by electrode E9to the adjacent tissue and the resulting evoked potential measured atelectrode E10 and, if desired, sequentially measured at electrodesE11-E16. Again, this process may be performed once or many times andaveraged. The baseline stimulate E9-evoked potential at E10 data isshown with a solid line in FIG. 9B. Electrodes E10-E16 may also be usedto supply stimulation pulses and evoked potentials may be measured atone or more of the other electrodes on lead 104.

It should be noted that evoked potentials may also/instead be measuredat additional or other electrodes 106 because the action potential maypropagate in a variety of directions depending on what nerves arestimulated and where they go. For example, evoked potentials may bemeasured at the electrodes 106 on the non-stimulating lead in additionto (or instead of) the electrodes on the stimulating lead. The deviationin voltage caused by the evoked potential due to the stimulation pulseat an electrode 106 on lead 102 could, for example, also be measured atone or more of the electrodes on lead 104 (either once or many times andaveraged) in order to provide additional baseline data and plots.Similarly, the deviation in voltage caused by the evoked potential dueto the stimulation pulse at an electrode 106 on lead 104 could also bemeasured at one or more of the electrodes on lead 102 (either once ormany times and averaged) in order to provide additional baseline dataand plots. There may also be instances where it is desirable to measurethe deviation in voltage caused by the evoked potential at thestimulating electrode. For example, the deviation in voltage caused bythe evoked potential due to a stimulation pulse at electrode E1 could bemeasured at electrode E1, and the deviation in voltage caused by theevoked potential due to a stimulation pulse at electrode E9 could bemeasured at electrode E9.

Evoked potential measurements may also be taken at various times afterthe baseline evoked potential measurements have been established foreach of the leads 102 and 104. For example, evoked potentialmeasurements may be taken at a periodic check-up or in response to anindication from the patient that the neurostimulation system is nolonger providing the same level of therapeutic effect that it did whenthe baseline measurements were taken. Such evoked potential measurementsare referred to herein as “subsequent” evoked potential measurements.The same stimulation pulse or one that provides a grossly equivalentintensity of stimulation effect (as identified by the patient) that wassupplied to tissue during the baseline measurement (e.g. 4 mA for 200μs) will be supplied by the same electrode and the deviation in thevoltage will be measured at the same electrode (or electrodes), eitheronce or many times and averaged, so that subsequent evoked potentialdata may be generated, measured and plotted.

For example, subsequent stimulate-E1/evoked-potential-at-E2 data may begenerated, measured, and plotted to determine whether there has beenmigration of lead 102, and subsequent stimulate E9-evoked potential atE10 data may be generated, measured and plotted to determine whetherthere has been migration of lead 104. If the data produces the sameevoked potential plots, or if the plots are within some acceptabledeviation therefrom, then it can be assumed that the leads have notmoved (or have moved an acceptably small amount). An unacceptabledeviation may, for example, be defined as an abrupt or significantshort-term change from the baseline evoked potential data. It is likelythat changes in the evoked potential data due to factors such asnecrosis or fibrosis change relatively slowly, whereas migration is morelikely to cause an abrupt change in the data. Accordingly, the baselineevoked potential measurements may include non-trended baseline evokedpotential measurements, where the baseline evoked potential values donot change over time, and trended baseline evoked potentialmeasurements, where the baseline evoked potential values are adjusted soas to account for factors such as tissue necrosis and fibrosis. Thesubsequent evoked potential measurements may be used in the mannerdescribed above in the context of impedance measurements to establishtrended baseline evoked potential values.

If, on the other hand, the subsequent evoked potential data is different(e.g., exhibits an abrupt or significant short term change from thebaseline evoked potential data) for one or both of the leads, or ifthere is no measurable evoked potential at the electrode where theevoked potential was measured for one or both of the leads, it can beassumed that the lead (or leads) associated with the different data hasmoved. For instance, if it is assumed that lead 102 has moved, theassumption may be checked by measuring the evoked potentials atelectrodes E3-E8 on lead 102 (either once or many times and averaged).If the subsequent plots associated with the evoked potentials atelectrodes E3-E8 were the same as, or some acceptable deviation from,the baseline plots, it may be assumed that the lead 102 has not movedand that there has been a change in conditions at electrode E2. Theassumption may also be checked by measuring the evoked potentials atelectrode E1 on lead 102 and/or electrodes E9-E16 on lead 104 (eitheronce or many times and averaged) if these electrodes were used for thebaseline measurements taken with respect to stimulation at electrode E1.Here too, if the subsequent plots are the same as, or some acceptabledeviation from, the baseline plots, it may be assumed that lead 102 hasnot moved. With respect to the tissue adjacent to electrode E2, thedifference in evoked potentials that is not due to lead movement couldbe due to scarring, necrosis or fat build up. Alternatively, there couldbe damage to electrode E2.

The same checking procedure may also be performed with respect to lead104. Specifically, the evoked potentials resulting from stimulation atelectrode E9 on lead 104 may be measured at electrodes E11-E16.Additionally or alternatively, if they were part of the baselinemeasurement process, the evoked potentials may be measured at electrodeE9 on lead 104 and/or electrodes E1-E8 on lead 102.

Referring to FIGS. 8B, 9A and 9B, lead 102 is shown having moved adistance corresponding to one electrode and, accordingly, the subsequentstimulate-E1/evoked-potential-at-E2 data is different. Lead 104, on theother hand, has not moved and the subsequent stimulate E9-evokedpotential at E10 data is essentially the same.

Evoked potentials may also be used to determine the direction andmagnitude of the movement. For instance, a feature comparison analysis(implemented by, for example, a cross-correlation technique) may be usedto determine the magnitude and direction of any shift. Here, stimulationenergy is individually provided to electrodes other than the originallystimulating electrode on the lead that has moved, and the evokedpotentials are measured at the other electrodes. The object is toidentify an electrode that produces similar evoked potential data as wasproduced by an electrode used in the baseline measurements. If such anelectrode is identified, it may be assumed that it now occupies theposition of the electrode that was used to produce the baseline data.With respect to the movement of lead 102, and as illustrated FIG. 8B and9A, the stimulate-at-E2/evoked-potential-at-E3 plot closely correspondsto the baseline stimulate-at-E1/evoked-potential-at-E2 plot (FIG. 9)because electrodes E2 and E3 are located in the positions previouslyoccupied by electrodes E1 and E2, respectively.

A flow chart that summarizes both of the methods described above ispresented in FIG. 10. In step 200, baseline artifactual tissue data,such as tissue impedance data or evoked potential data, is measured withone of the leads. Next, in step 210, artifactual tissue data is measuredat one or more subsequent times with the same lead. Finally, in step220, the subsequent artifactual tissue data is compared to the baselineartifactual data (which may included non-trended baseline artifactualdata and/or trended baseline artifactual data) to determine whether thelead has moved. This process may be repeated with each lead.

III. Exemplary Corrective Measures

The corrective action that may be taken after it has been determinedthat one or more of the leads in a neurostimulation system (such as anSCS system) has moved generally falls into two categories—(1) surgicalremoval or repositioning and (2) reprogramming. Surgical removal orrepositioning will typically be employed when it has been determinedthat one or more of the leads has moved too far to make reprogramming aviable option. If, for example, the therapeutic regimen required that anelectrode be located in the baseline location of electrode E2 on lead102 (FIG. 6A), the therapeutic regimen could not be performed once lead102 migrated to the location shown in FIG. 6B because there is no longerany electrode in that location. Surgical removal may also be required ifone or more of the electrodes are damaged or fail.

With respect to reprogramming, individualized information concerning theactual movement (or lack of movement) of each lead will allow thereprogramming to proceed in a far more efficient manner than would bethe case if the entity tasked with reprogramming (i.e. a physician orthe neurostimulation system) merely knew that at least one of the leadshas moved because the relative positions of the leads has changed.Assuming for example that the leads 102 and 104 illustrated in FIG. 6Awere employed in a therapeutic regimen that involve sourcing and sinkingstimulation pulses from electrodes E4, E5 and E6 on lead 102 andelectrodes E13 and E14 on lead 104. After lead 102 moved to the positionillustrated in FIG. 6B, and it was determined by the present inventionsthat only lead 102 moved and that lead 102 moved toward the IPG 110 adistance corresponding to two electrodes, the therapeutic regimen mayreprogrammed by simply substituting electrodes E2, E3 and E4,respectively, for electrodes E4, E5 and E6.

Reprogramming may be performed automatically or by a clinician.Automatic reprogramming, which is especially useful when lead migrationis being continuously monitored, could be truly automatic (i.e. it wouldhappen without the patient's knowledge). Alternatively, the IPG 110could provide the patient with an indication that at least one lead hasmoved and give the patient the option of trying the automaticallyreprogrammed stimulation regimen or simply reporting the lead migrationto the clinician. Reprogramming by the clinician, either in response toa notification from the IPG 110 or patient complaint, would typicallyinvolve allowing the external programmer 118 to modify (or simplysuggest a modification of) the therapeutic regimen based on the leadmigration data from the IPG 110. Alternatively, the lead repositioningis recorded for the clinician to review for use during reprogramming,thereby reducing the amount of clinician time (and expense) required toreprogram the therapeutic regimen as well as the likelihood that anexpensive fluoroscopic procedure will be required.

Although the inventions disclosed herein have been described in terms ofthe preferred embodiments above, numerous modifications and/or additionsto the above-described preferred embodiments would be readily apparentto one skilled in the art. By way of example, but not limitation, thepresent inventions include neurostimulation systems that also compriseat least one neurostimulation lead. It is intended that the scope of thepresent inventions extend to all such modifications and/or additions andthat the scope of the present inventions is limited solely by the claimsset forth below.

We claim:
 1. A neurostimulation system for use with an individualimplantable neurostimulation lead, the neurostimulation systemcomprising: measurement circuitry configured for measuring baselineartifactual data concerning tissue in the vicinity of the individualneurostimulation lead using only the individual neurostimulation leadand an implantable pulse generator, and measuring subsequent artifactualdata concerning the tissue in the vicinity of the individualneurostimulation lead at a subsequent time with only the individualneurostimulation lead and the implantable pulse generator; and controlcircuitry configured for determining a magnitude that the individualneurostimulation lead has migrated from a baseline location by comparingthe baseline artifactual data to the subsequent artifactual data,wherein the baseline location is a location of the individualneurostimulation lead during the measuring of the baseline artifactualdata.
 2. A neurostimulation system as claimed in claim 1, wherein theartifactual data comprises tissue impedance data.
 3. A neurostimulationsystem as claimed in claim 1, wherein the artifactual data comprisesevoked potential data.
 4. A neurostimulation system as claimed in claim1, wherein the control circuitry is further configured for takingcorrective action in response to a determination that the individualneurostimulation lead has migrated.
 5. A neurostimulation system asclaimed in claim 4, wherein taking corrective action comprisesreprogramming a therapeutic regimen in response to a determination thatthe individual neurostimulation lead has migrated.
 6. A neurostimulationsystem as claimed in claim 1, wherein the control circuitry is furtherconfigured for maintaining the baseline artifactual data measurement asa non-trended baseline artifactual data measurement.
 7. Aneurostimulation system as claimed in claim 1, wherein the controlcircuitry is further configured for creating a trended baselineartifactual data measurement based on the baseline artifactual datameasurement.
 8. A neurostimulation system as claimed in claim 7, whereinthe trended baseline artifactual data measurement is based on adifference between the baseline artifactual data measurement and thesubsequent artifactual data measurement.
 9. A neurostimulation system asclaimed in claim 8, wherein the trended baseline artifactual datameasurement is created by replacing the baseline artifactual datameasurement with a new baseline artifactual data measurement if thedifference is relatively small, and maintaining the baseline artifactualdata measurement if the difference is relatively large.
 10. Aneurostimulation system as claimed in claim 9, wherein the new baselineartifactual data measurement is the subsequent baseline artifactual datameasurement.
 11. A neurostimulation system as claimed in claim 9,wherein the new baseline artifactual data measurement is an average ofthe baseline artifactual data measurement and the subsequent baselineartifactual data measurement.
 12. A neurostimulation system as claimedin claim 7, wherein the trended baseline artifactual data measurement isa moving average that includes the baseline artifactual data measurementand the subsequent baseline artifactual data measurement.
 13. Aneurostimulation system as claimed in claim 1, wherein the controlcircuitry is further configured for determining a direction in which theneurostimulation lead has migrated from a baseline location by comparingthe baseline artifactual data to the subsequent artifactual data.
 14. Aneurostimulation system as claimed in claim 1, further comprising atleast one stimulation source.